In helicopters which use gas turbine engines, the acceleration of the engine may be controlled by signals from both the cockpit (e.g. the collective pitch signal) and the supervisory electronic control (e.g. the Digital Electronic Control output). These two signals control the volume of fuel delivered from the Hydromechanical Unit (HMU) to the fuel nozzles in the engine, thus controlling engine acceleration.
The HMU normally includes limiting electronics or mechanical apparatus which prevent the acceleration of the gas turbine engine from exceeding predetermined limits. This limit may be referred to as the acceleration schedule or accel schedule. In many applications, the Accel Schedule is implemented by, for example, mechanical cam shafts which prevent the fuel flow from exceeding predetermined limits for various engine conditions. The HMU is adapted to receive a number of signals, including the DEC output and the collective output and to generate therefrom a "fuel demand signal". The fuel demand signal is used by the HMU to control the flow of fuel to the engine.
Normally, the fuel demand signal is not limited by the accel schedule. However, the fuel demand inputs to the HMU may combine to form a fuel demand signal that exceeds accel schedule. In this case the HMU controls fuel flow according to the Accel Schedule even if the fuel demand signal continues to increase. However, changes in engine conditions and/or control inputs may result in a decrease in fuel demand. In an ideal system, when the fuel demand decreases the HMU would respond immediately, delivering fuel according to the demand. However, delays in the controls, the engine and the HMU normally result in a delay between the time when the fuel demand signal decreases and the time when the HMU responds. It would, therefore, be advantageous to provide a system wherein the delay between a decrease in the fuel demand signal and an actual decrease in fuel flow is reduced.